Processor bus bridge security feature for network processors or the like

ABSTRACT

Described embodiments provide a system having a bridge for communicating information between two processor buses. The bridge receives a command from a first bus, the command having an identification field and an address field. As the command is entered into a buffer in the bridge, the address field is checked against one or more addresses. If there is a match, then control bits are checked to see if the command will be allowed or not depending on the identification field value. If the command is not transferred to the second bus, a flag is set in the buffer, and an error message is returned to the first bus, and an interrupt may be generated. The control bits allow commands access to specific addresses on the second bus or deny the access depending on the command identification field. Bit-wise masking provides a range of values for identification and address field matching.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.provisional application No. 61/313,196 filed 12 Mar. 2010 the teachingsof which are incorporated herein in their entireties by reference.

The subject matter of this application is related to U.S. patentapplication Ser. Nos. 12/430,438 filed 27 Apr. 2009, 12/729,226 filed 22Mar. 2010, 12/729,231 filed 22 Mar. 2010, U.S. patent application Ser.Nos. 12/782,379, 12/782,393, and 12/782,411, all of which were filed on18 May 2010, and U.S. patent application Ser. Nos. ______ and ______,all of which were filed concurrently with this application, theteachings of all of which are incorporated by reference herein in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processor systems generally and, morespecifically, to a local bus bridge for interfacing between differentprocessor bus architectures.

2. Description of the Related Art

Network processors are generally used for analyzing and processingpacket data for routing and switching packets in a variety ofapplications, such as network surveillance, video transmission, protocolconversion, voice processing, and internet traffic routing. Early typesof network processors were based on software-based approaches withgeneral-purpose processors, either singly or in a multi-coreimplementation, but such software-based approaches are slow. Further,increasing the number of general-purpose processors had diminishingperformance improvements, or might actually slow down overall NetworkProcessor throughput. Newer designs add hardware accelerators to offloadcertain tasks from the general-purpose processors, such asencryption/decryption, packet data inspections, etc.

Because of the complexity of network processors and other system-on-chip(SoC) architectures, there is usually more than one kind of processorbus implemented on the chip, especially if there is a mixture ofprocessor designs on the chip. Generally, one or more processors tocommunicate with external peripherals, memory, or each other using aprocessor bus. However, processor buses are not standardized and eachprocessor design utilizes a different, incompatible, bus structure andprotocol. Nonetheless, data and addresses need to be passed betweenbuses using a bus “bridge”.

It might be desirable to limit access to data or software accessiblefrom one bus by clients on the other bus. For example, critical bootcode, encryption keys, or message authentication keys stored in memorycoupled to one bus, but accessible from the other bus through thebridge, should be protected from unauthorized or inadvertent access oroverwriting through the other bus. Thus, it is desirable to provide abus bridge that interfaces between two different buses, the bridgehaving security features for controlling access to memory or otherhardware, such as input-output ports, control registers, etc., coupledto one bus, from activities on the other bus.

SUMMARY OF THE INVENTION

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

Described embodiments provide a bridge coupled to a first bus and asecond bus and having an access table with at least one entry comprisingan address and an identification value. The bridge receives a commandfrom the first bus, the command having an identification field and anaddress field. The bridge compares the address field of the commandagainst the address in the access table entry to generate an addressmatch if the address field and the address match, and compares theidentification field against the identification value in the accesstable entry to generate an identification match if the identificationfield and the identification value match. The received command istransmitted over the second bus if there is an address match and thereis an identification match, and an error response is transmitted overthe first bus if there is an address match and no identification match.

An alternative described embodiment provides a bridge coupled to a firstbus and a second bus, and has an access table, a receiver, a comparator,a first transmitter, and a second transmitter. The access table has atleast one entry comprising an address and an identification value. Thereceiver, coupled to the first bus, receives a command from the firstbus having an identification field and an address field. The comparator,coupled to the receiver and the access table, compares the address fieldof the received command against the address in the access table entry togenerate an address match if the address field and the address match,and adapted to compare the identification field against theidentification value in the access table entry to generate anidentification match if the identification field and the identificationvalue match. The first transmitter, coupled to the second bus and thereceiver, transmits the received command over the second bus if there isan address match and there is an identification match. The secondtransmitter, coupled to the first bus, transmits an error response overthe first bus if there is an address match and no identification match.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention willbecome more fully apparent from the following detailed description, theappended claims, and the accompanying drawings in which like referencenumerals identify similar or identical elements.

FIG. 1 shows a block diagram of a network communications processor;

FIG. 2 shows a block diagram of the network communications processor ofFIG. 1 with additional details;

FIG. 3 shows a block diagram of an exemplary communication bus of thenetwork communications processor of FIG. 2;

FIG. 4 shows a simplified interface diagram of the AXI-to-PLB bridge ofthe network communications processor of FIG. 3;

FIG. 5 illustrates the AXI-to-PLB bridge of FIG. 4 in more detail and inaccordance with embodiments of the present invention;

FIG. 6 shows a simplified block diagram of a block within the AXI-to-PLBbridge of FIG. 5, in accordance with embodiments of the presentinvention;

FIG. 7 shows a block diagram of a statistical data block in theAXI-to-PLB bridge of FIG. 4, in accordance with embodiments of thepresent invention;

FIG. 8 is a simplified flow chart of initializing and operation of thestatistical data block of FIG. 7;

FIG. 9 is a simplified block diagram of a write command security blockin the AXI-to-PLB bridge of FIG. 5;

FIG. 10 is a simplified block diagram of a read command security blockin the AXI-to-PLB bridge of FIG. 5; and

FIG. 11 is a simplified flowchart illustrating an exemplary operation ofthe security block of FIGS. 9 and 10 for allowing or disallowing accessby the AXI bus of the PLB in FIG. 4.

DETAILED DESCRIPTION

Table 1 defines a list of acronyms employed throughout thisspecification as an aid to understanding the described embodiments ofthe present invention:

TABLE 1 USB Universal Serial Bus FIFO First-In, First-Out SATA SerialAdvanced Technology I/O Input/Output Attachment SCSI Small ComputerSystem Interface DDR Double Data Rate SAS Serial Attached SCSI DRAMDynamic Random Access Memory PCI-E Peripheral Component Interconnect MMBMemory Manager Block Express SoC System-on-Chip MAI MicroprocessorAccess Interface AXI Advanced eXtensible Interface PLB Processor LocalBus AMBA Advanced Microcontroller Bus MPP Modular Packet ProcessorArchitecture AWID Write Request (Address) ID PPCS Power PC ComponentSubsystem FIFO First-In-First-Out memory AWADDR AXI write command dataaddress ARID Read Request (Address) ID AWLEN AXI write command data(burst) length BRESP Write Response Status AWSIZE AXI write command data(burst) size IP Internet Protocol CRC Cyclic Redundancy Check TCPTransmission Control Protocol UDP User Datagram Protocol WID Write DataID RID Read Response (Data) ID PHC PLB Hang Detect Control Block PTC PLBTimer Control Block PWC PLB Write Control Block PRC PLB Read ControlBlock PCC PLB Command Control AWC AXI Write Control Block ARC AXI ReadControl Block WB Write Buffer RB Read Buffer BTAG WTAG of completingwrite request A2P AXI bus-to-PLB RTAG Read Request Index Tag BID WriteResponse ID WTAG Write Request Index Tag DCR Device Control Block WASCWrite Access Security Check Block SMID Special Master ID SMUID SpecialMaster User ID TADDR Target Address WASCF Write Access Security CheckFlag

FIG. 1 shows a block diagram of an exemplary single-chip networkprocessor system implemented as a system-on-chip (SoC), NetworkProcessor 100. Network Processor 100 might be used for processing datapackets, performing protocol conversion, or the like. Reference hereinto “one embodiment”, “an exemplary embodiment”, or “an embodiment” meansthat a particular feature, structure, or characteristic described inconnection with the embodiment can be included in at least oneembodiment of the invention. The appearances of the phrase “in oneembodiment” in various places in the specification are not necessarilyall referring to the same embodiment, nor are separate or alternativeembodiments necessarily mutually exclusive of other embodiments. Thesame applies to the term “implementation.”

Network processor 100 includes on-chip shared memory 110, one or moreinput-output (I/O) cores, shown as I/O core 112, one or moremicroprocessor (pp) cores, shown as μP cores 114 ₁-114 _(M), and one ormore engine cores 120 ₁-120 _(N), where M and N are integers greaterthan 1. Network Processor 100 also includes task ring 130, memorymanager block (MMB) 140, MMB ring 116, and external memory interface 150for communication with external memory 160. External memory 160 mighttypically be implemented as a dynamic random-access memory (DRAM), suchas a double-data-rate three (DDR-3) DRAM, for off-chip storage of data.In some embodiments, such as shown in FIG. 1, all of the one or moreI/O, μP and engine cores, and MMB 140, are directly coupled to sharedmemory 110. In alternative embodiments, each of the one or more I/O, μPand engine cores, and MMB 140 might not need to be directly coupled toshared memory 110. For example, as described in greater detail withregard to FIG. 2, each of the one or more I/O, μP and engine cores, andMMB 140 (collectively referred to as “cores”) might be coupled to aswitch system that is then coupled to shared memory 110.

Shared memory 110 might include one or more FIFO queues 142. Asdiscussed in more detail below, one or more FIFO queues 142 might bedynamically allocated in shared memory 110 to the various cores 112,114, and 120 based on corresponding requests by the cores to MMB 140.Each core might request allocation of memory for additional FIFO queuesvia MMB ring 116. While, in FIG. 1, task ring 130 and MMB ring 116 areshown coupled to the various cores in an order, one skilled in the artwill realize that such order is for illustrative purposes only as an aidto describing the present invention, and any one of the cores might beadjacent to another core along rings 130 and 116. As described herein,the term “adjacent” describes either a previous core or subsequent coreon communication rings 116 and 130. For example, in the exemplaryembodiment shown in FIG. 1, MMB 140 is adjacent to μP core 114 _(M) andI/O core 112.

I/O core 112 might typically be implemented as hardware that connectsNetwork Processor 100 to one or more external devices through I/OCommunication link 102. I/O Communication link 102 might generally beemployed for communication with one or more external devices, such as acomputer system or networking device, that interface with NetworkProcessor 100. I/O Communication link 102 might be a custom-designedcommunication link, or might conform to a standard communicationprotocol such as, for example, a Small Computer System Interface(“SCSI”) protocol bus, a Serial Attached SCSI (“SAS”) protocol bus, aSerial Advanced Technology Attachment (“SATA”) protocol bus, a UniversalSerial Bus (“USB”), an Ethernet link, an IEEE 802.11 link, an IEEE802.15 link, an IEEE 802.16 link, a Peripheral Component InterconnectExpress (“PCI-E”) link, a Serial Rapid I/O (“SRIO”) link, or any otherinterface link. Received packets are preferably placed in memory 110 andthen one or more “tasks” corresponding to the received packets areprovided, via task communication ring 130, to one or more of the variouscores. As described herein, a task is a command issued between cores toperform processing functions on at least a portion of packet data.Transmitted packets are preferably received from a task and transmittedexternally.

Task ring 130 is a communication bus linking adjacent cores together ina serial or “daisy-chain” fashion. In one embodiment, task ring 130might be a unidirectional ring that passes task control information froma source core to a destination core, and the tasks might contain addresspointers to data stored in shared memory 110. As described herein, tasksare instructions to the destination core to perform certain functions.Tasks received by a destination core might be stored in a correspondingone of FIFO queues 142, and the data corresponding to the task to beprocessed by the destination core might be stored in shared memory 110.

Tasks allow Network Processor 100 to process a wide variety of data andcontrol messages more efficiently than network processors with a fixedpipeline or non-pipelined architecture. As discussed in more detail inthe incorporated patent applications, the sequence of the tasks dependson i) the type of packet and ii) the type of processing performed by thevarious cores on a particular packet (or group of packets), controlmessage, or other data. This is referred to herein as a “VirtualPipeline™”, a trademark of LSI Corporation, of Milpitas, Calif.

In embodiments of the present invention, a virtual pipeline operates byeach core receiving a task, executing that task, and assigning asubsequent task to another (or the same) core depending on the packet orinstruction being processed. For purposes here, a core generating a taskis referred to as a source core, and a core given a task to execute isreferred to as a destination core. Tasks provided to a destination coreare written to shared memory 110 by the source core and read from sharedmemory 110 by the destination core. Task ring 130 provides acommunication path for the various cores to pass tasks to each otherutilizing messages that contain the address pointers to datacorresponding to the task stored in shared memory 110. Although shown inFIG. 1 as a ring bus, it is understood that other topologies other thana ring might be used to pass tasks from core to core, such as directconnections between each of the cores or use of a switch system. Asdescribed below, a task data structure might typically include i) anidentification of a virtual pipeline for the task, ii) packet specificparameters and engine instructions for the virtual pipeline, iii) inlineheader and trailer data for the task, and iv) pointers to data stored inmemory 110.

External memory interface 150 couples shared memory 110 to external DRAM160 to provide off-chip storage of data not needed by the various enginecores 120 ₁-120 _(N) and μP cores 114 ₁-114 _(M) to free space in sharedmemory 110. Memory management block (MMB) 140 allocates and frees memoryresources in shared memory 110. Memory is allocated for suchapplications as task FIFO storage (e.g., FIFOs 142), packet datastorage, hash-table collision handling, timer event management, andtraffic manager queues. MMB 140 provides reference counts to each blockof memory within shared memory 110. Multiple reference counts allow formore efficient storage of information, such as multicast traffic (datato be sent to multiple destinations) or for retransmission. Multiplereference counts remove the need for replicating the data each time thedata is needed. MMB 140 preferably tracks the memory allocations using astack-based approach since a memory block recently released ispreferably the next block to be allocated for a particular task,reducing cache trashing and cache tracking overhead.

FIG. 2 is a simplified diagram of the Network Processor 100 of FIG. 1with additional details. Like-numbered items perform analogously withthose described with respect to FIG. 1 and are not discussed herein inconnection with FIG. 2 except as noted. Non-blocking crossbar switch 202connects cores 112, 120 ₁-120 _(N), 114 ₁-114 _(M) and MMB 140 to systemcaches 204 in shared memory 110, and a connection made throughnon-blocking crossbar switch 202 is considered, for purposes here, adirect connection between a core and the memory 110. Crossbar switch 202is preferably a non-blocking switch arbitrating memory access priorityby the cores. As described in more detail below, system caches 204 areaddressed via the crossbar switch in such a way as to balance access tothe caches, referred to herein as striping, helping to avoid hot spotsin shared memory 110, improve performance of the caching in and out ofexternal memory 160, and reduce cache access bottlenecks. Thus, inembodiments of the present invention, each system cache 204 might form amemory array, and the number of system caches might preferably beimplemented as a power of two. Non-blocking crossbar switch 202 might beimplemented such as described in the above-identified related U.S.patent application Ser. Nos. 12/430,438 filed 27 Apr. 2009, 12/729,226filed 22 Mar. 2010, and 12/729,231 filed 22 Mar. 2010.

In embodiments of the present invention, there are three types of“clients” (e.g., types of cores) supported by the crossbar: i) processorcores, ii) I/O cores, and iii) general clients. Processor cores 114₁-114 _(M) access the system cache via the crossbar and have generallyabsolute priority to reduce to a minimum latency for the processors thatmight otherwise suffer from stall cycles when latency increases beyond athreshold.

I/O cores are a type of client that support I/O interfaces, such as I/Ocore 112, and have medium memory access priority. Allocation of thispriority level is important because these types of clients might sufferfrom underrun/overrun issues when supporting their interfaces. The I/Ocore clients generally experience higher latency than general processorcores, but lower latency than general clients. However, since the I/Ofunction is generally easy to read-ahead and write-behind, this higherlatency for these types of clients causes little or no impact to overallperformance. If the maximum latency is bounded, the size of theread-ahead and write-behind required might be determined.

General clients have the lowest access priority for shared memory 110.General clients might constitute all other modules of Network Processor100. For example, these modules might be hardware accelerators orspecial purpose processors. One such special purpose processor, shown inFIG. 3, is Microprocessor Access Interface (MAI) 302. As shown in FIG.3, MAI 302 might provide a physical bus interface to enable virtualpipeline connections between one or more internal and externalprocessors and non-blocking crossbar switch 202. All general clientfunctions are designed to be tolerant of the memory latency and cantolerate short disruptions in latency without causing performanceissues.

MAI 302 is generally performance insensitive to absolute latency. Thus,MAI 302 is typically operated with high utilization at the lowestpossible priority value. The I/O core clients are sensitive to memoryaccess latency, but only to having predictable latency. If the memoryaccess latency is bounded, prefetch and write behind might be employedto operate without underrun or overrun. In general, all other memoryclients are sensitive only to average latency. In addition, they mightprefetch and write-behind to achieve a desired level of performance.

FIG. 3 shows a block diagram detailing an interface 300 betweenprocessors 114 ₁-114 _(M) and crossbar switch 202 and externalprocessors or other off-chip peripherals. As shown, Network Processor100 contains one or more microprocessors 114 ₁-114 _(M), each of whichmight include internal L1/L2 caches (not shown). The multiple processorsimplement cache coherency between themselves as is well known in theart. Cache misses from the processors go across the main system memorycrossbar 202 to on-chip shared memory 110 and, if necessary, externalmemory 160. Interface 300 might also include a standard set ofperipheral interfaces (USB, EEPROM, etc., not shown) on or off the chip.For example, as shown in FIG. 3, PCI-E interface 308 might be provided.PCI-E interface 308 allows for any external device operating inconformance with the PCI-E protocol to be attached to Network Processor100. For example, PCI-E interface 308 might allow one or more externalprocessors to access shared memory 110, task queues 142 and otherresources on the chip, such as task ring 130 and MMB ring 116. Thus, thetask communication mechanism employed by the internal processors canalso be used for message passing between the internal and externalprocessors.

Communication between processors 114 ₁-114 _(M) and memory 110 viacrossbar switch 202 occurs via two paths, one path for processorinstructions requiring latency less than a predefined threshold(“minimal latency”) and the other path for other types of information,such as tasks sent via the virtual pipeline, that are not as sensitiveto memory latency. Memory access interface (MAI) 302 handlesinstructions that are less sensitive to memory latency. MAI 302 is aninterface between crossbar switch 202 (and memory 110) and the internalprocessor(s) 114 ₁-114 _(M) and any attached processor(s) external toNetwork Processor 100. MAI 302 provides an interface for different kindsof microprocessors to receive and send tasks like any of the internalcores coupled to shared memory 110.

For example, MAI 302 might be coupled to PCI-E interface 308. Asillustrated here, MAI 302 connects to the crossbar 202 for itstask-based communications. For processor communications, MAI 302 iscoupled to AXI bus 304. The AXI protocol is a higher frequencyimplementation of the Advanced Microcontroller Bus Architecture (AMBA)introduced by ARM Ltd. A complete explanation of the AMBA architectureand the AXI bus operation can be found in “AMBA AXI ProtocolSpecification”, copyright 2003, 2004, ARM Limited, the contents of whichare incorporated herein in its entirety. AXI bus 304 enables MAI 302 tocommunicate with any peripheral device or processor that supports theAXI protocol. AXI-to-PCI-E controller 308 might be used by MAI 302 toallow external processors to access AXI bus 304. Internal processors 114₁-114 _(M) access AXI bus 304 via AXI-to-PLB Bridge 306. PLB is aproprietary microprocessor bus architecture introduced by IBM. Inembodiments of the present invention, processors 114 ₁-114 _(M) arePower PC® processors that utilize PLB 310 for communication betweenprocessors and crossbar 202. AXI-to-PLB (A2P) bridge 306 allowsprocessors 114 ₁-114 _(M) to communicate with AXI bus 304, and thusexternal processors (via AXI-to-PCI-E controller 308).

For purposes here and as used herein, the AXI bus and PLB are internalsystem buses hierarchically comprised of sub-components. For the AXI,the next lower layer of bus hierarchy is referred to as a “channel” or“path”. For the PLB, the next lower layer of bus hierarchy is sometimesreferred to as an interface, but for purposes here, the term “channel”or “path” is used here as well.

MAI 302, AXI-to-PCI-E controller 308 and AXI-to-PLB Bridge 306 make thearchitectural design of Network Processor 100 “agnostic” to the chosenprocessor architecture for processors 114 ₁-114 _(M) or any externalprocessors. For example, a given design might employ Pentium® or PowerPC® processors, or a combination of different processors, withoutchanging the interfaces to various software modules of Network Processor100. Pentium® is a registered trademark of Intel Corporation, and PowerPC® is a registered trademark of IBM.

As mentioned above, task communication within the architecture ofNetwork Processor 100 is handled by MAI 302. MAI 302 allows theprocessors to send and receive tasks to and from other processors orother cores within the architecture. MAI 302 is also used forconfiguration updates. All accesses that MAI 302 makes to memory 110 arefully coherent with the processor receiving or issuing a task. If anexternal processor supports I/O coherency for PCI-E transactions, thenthe external processors are coherent as well. All interactions with MAI302 are preferably non-blocking to the processor and generally involveonly local memory updates. Processor overhead is reduced to a few cachedmemory accesses (no I/O cycles required). Advantageously, MAI 302 allowsthe programming model might be substantially the same for internal andexternal CPUs. The programming model might also be substantially thesame for special purpose processors, such as digital signal processors.

Multiple task queues (e.g., FIFOs 142 of FIG. 1) for each processor aresupported for input, output, and processor configuration. This allowsindependent access to the queues from different processors or cores. Anordered queue process can be used to provide per flow load balancingamong the cores. Tasks can also be directed to a specific queue.

One component of MAI 302 manages processor addressable FIFO buffers. Foroutgoing tasks, MAI 302 will copy the data out of theprocessor-specified buffer quickly. Therefore, the processor will beable to reuse the buffers quickly. The task queues going towards theCPUs can also be quite shallow, for example, the queues might only bedeep enough for MAI 302 to have sufficient time to push a next task into the buffer. Buffer management for task-receive queues, explained inmore detail below, is handled by the processor providing MAI 302 with asmall set of buffer addresses. This approach might require an extra copyof the data being buffered, but this copy might have only a smallperformance impact because of the aggregate bandwidth provided by thememory subsystem and the intelligent management of the cache. No extraexternal memory access cycles will be required for this copy.

Turning to FIG. 4, an AXI bus-to-PLB (A2P) top-level interface diagramis shown to provide details regarding signal flow between the AXI bus304 and PLB 310 and a brief description of the purpose of various busesbetween the AXI bus 304 and the bridge 306, as well as between PLB 310and the bridge 306. The AXI bus is a subsystem 304 having aninterconnect fabric 402 for passing information from and to AXI busclients such as those shown in FIG. 3. Further details on theinterconnect fabric 402 can be found in the aforementioned AXI ProtocolSpecification. Similarly, the PLB 310 is a subsystem having a PLBcontroller 404, a proprietary SoC IP block instantiated within the PowerPC Component Subsystem (PPCS) available from IBM. The bus controller 404arbitrates address and command information, along with data read fromthe PLB, passing through the controller 404. In this embodiment and aswill be described in more detail below, write requests originating fromthe AXI bus 304 are transmitted to the bridge 306 via AXI WADDR channel408 and write data over AXI WDATA channel 406 and are assembled in theWrite Buffer block (WB) 410. Data to be written, stored in WB 410, issent to the PLB controller via channel 412 and the write address overchannel 414. Read requests over AXI RADDR channel 416 originating fromthe AXI bus 304 are assembled in the read buffer block (RB) 418 and sentto the PLB controller over channel 414. In response, data from the PLB310 are received over channel 422, assembled in the RB 418, and sent tothe AXI bus via channel 424. Details regarding the channels can be foundin the aforementioned AXI Protocol Specification. For purposes here, theAXI system bus protocol is burst based. Every transaction has addressand control information on either channel 408 or 416 that describes thenature of the request for data to be transferred. The controlinformation includes the number of beats of a burst and the size in bitsof the beats. Also included are request IDs to individualize requestsand provide a means for identifying the data transfer and requestculminating response phases that are to be correlated with suchparticular request.

But the Data transfer phases also include control information. Each beatof channel 406 write data is accompanied by write strobe information aswell as the write data ID and last status. Each beat of read data 416 isaccompanied by read response status, a read data ID and a last beatflag. If a write is to occur, then write data from the AXI bus isprovided over channel 406 and, in turn, to the PLB over channel 412.When a read request is executed, data from the PLB over channel 422 ispassed to the AXI bus over channel 424. Channel 426 is used to conveythe write response status of AXI control signals described in section2.4 of the above-referenced AMBA AXI Protocol Specification.

Only the first and last beats of a data burst may be less than aquad-word on the PLB subsystem 310. The PLB command's address isadjusted if the leading bytes of a quad-word are not to be requested aspart of the first beat. The PLB command's byte enable indicates whichbytes are to be used for the last beat of the burst.

However, the AXI bus 304 allows bursts using various sizes with eitherquad-word or smaller than quad-word beats. Thus the bridge 306 will haveto perform byte lane reordering and beat transfer size transformationsfor both read and write transfers. In some cases this will also requiresplitting of some of these AXI transfers into multiple PLB transfersbecause the AXI burst length can exceed the maximum PLB burst length.For write transfers, the byte count is not known until the last beat ofthe transfer is received from the AXI bus. Splitting of PLB commands maybe necessary to prevent commands from crossing PLB 128 byte boundaries.

On the PLB side of the bridge 306, channel 428 conveys the status ofvarious requests sent to controller 404, such as read and writerequests, sent over channel 414 to PLB controller 404 in response to AXIbus-initiated read and write requests. Bidirectional channel 430 handleshang detection and resolution. It provides a means for the bridge 306 toassert that it is not making forward progress with commands to the PLB.The bridge 306 maintains per WTAG and RTAG progress counters insub-block 534, described in more detail below, that increment inresponse to a PLB system count increment pulse when the bridge 306 hasoutstanding AXI requests. Channel 430 also provides a control signalthat bridge 306 monitors and, when that control signal is asserted, thebridge 306 will not issue any new commands via PLB command channel 414,with the exception of the case where the bridge 306 is asserting that itis in the hung state it may re-issue commands that have been previouslyissued for which a combined response of retry was received. Channel 428conveys the combined response of the PLB slaves to the bridge 306 toissued PLB read and write commands. The combined response can be any oneof acknowledge, retry, or error. The slave-tag and slave-ID are providedto the bridge 306 to allow the bridge 306 to identify each beat of writedata onto the channel 412 during writing of data from the bridge 306 tothe bus controller 404.

The slave tag is given to a PLB master as part of the combined responseon channel 428 to a write command. The slave tag originates in the slavetargeted by the master and is passed through the PLB Bus Controller 404to the bridge 306. The bridge 306 provides the slave tag along with thedata in the PLB write data transfer phase. This allows the target PLBslave to match the transferred data with the earlier command. The slaveID is given to the master as part of the combined response 428 to awrite command. This slave ID originates in the PLB controller 404. Thebridge 306 provides this slave ID along with the data in the PLB writedata phase, so that the PLB controller 404 can route the write data tothe correct target slave.

Reset signals and various clocks for the bridge 306 are present onchannel 432. Control, configuration, and other data are carried overchannel 434 from and to Device Control Register (DCR) arbiter 436, partof the PLB structure. The arbiter 436 arbitrates access of the DCRmasters to the DCR bus slaves of which the bridge 306 is a DCR slave.Internal configuration and status registers in the bridge 306 areaccessible by DCR masters.

In this exemplary embodiment, the bridge 306 is an AXI bus “slave” and aPLB “master”, i.e., bridge 306 does not initiate operations on the AXIbus 304 but does initiate operations on the PLB 310. It is understoodthat the bridge 306 may be an AXI bus master and a PLB slave or a masteron both buses.

Upon reset of the Network Processor 100 (FIG. 1), all internal statemachines, control registers, data registers, and FIFO memories arecleared in the bridge 306. Following reset, the bridge 306 should be inan operational configuration, but the General Control and Statusregisters (GCS) 440 may be modified over the Device Control RegisterSlave channel (DCR_S) 434 to reconfigure the bridge 306.

FIG. 5 illustrates the bridge 306 in more detail. The functions of thevarious blocks in the bridge 306 are described herein.

The AXI Write Control (AWC) block 502 interfaces to the AXI slave writechannels 406, 408, 426, to accept write requests (commands), writeaddress and write data from the AXI bus 304, and communicates writechannel responses to the AXI BRESP 426. For purposes here and as usedherein, the terms “commands” and “requests” are used interchangeably. Asexplained in more detail below, the AW, W, and B FIFOs in AWC 502provide clock domain crossing, rate adaptation, and some buffering forthe WADDR (408), WDATA (406), and BRESP (426) write channels. The WriteRequest Memory 504 stores AXI Write Channel requests by entries indexedby WTAG indexes from the AWID block 510, described below. The memory 504is may be implemented as a group of register bit, vector, and counterarrays and is a memory indexed, in this embodiment, with values rangingfrom zero to fifteen. Thus the memory 504 is divided into sixteenportions. Each portion represents a potential write request trafficcontext comprising an AXI write request from bus 408, the beats of writedata and write strobe information from bus 406, the status and progressof the request. Different requests can simultaneously proceed throughvarious pipeline states from clock cycle to clock cycle. As will beexplained in more detail in connection with FIG. 6, when a new writerequest is presented to the AWC 502, the vacancy status of the writerequest memory 504 is checked for a free WTAG index, and for the newcommand's write address (AWADDR) and AWID, one portion identifying theAXI maser, the other portion used by the AXI master to distinguish oneof its requests from another having been already present. If the AWID isalready present, or if there are no free WTAG indexes, the request mustwait in the AW FIFO for the corresponding WTAG entry to be de-allocated.

Otherwise, having passed the admissibility check in AWID block 510operating upon contents of memory 504, as described below in connectionwith FIG. 6, the candidate request in FIFO 514 will be admitted and itscorresponding request information stored in memory 504.

If WID check and WTAG lookup 506 and AWID block 510 (both described inmore detail below) determine if the new AXI write request write data inFIFO 508 to correspond to an outstanding request of a context stored inmemory 504, then the AXI Write Data, WID, and Write Strobes from bus 406are popped from the W FIFO 508. Memory 504 also contains a per WTAGindexed Boolean status that indicates if the last beat of a context'swrite request write data has been received, and WID check block 506takes this into consideration in order that PLB memory boundariesbetween request write data be conserved. Each pop of the W FIFO 508reads one beat that had been transferred to the W FIFO from the AXIWrite Data channel 406. The popped WID is used to lookup the WTAG indexby matching the stored AWID. The AWC 502 maintains per WTAG currentaddress and beat counters in memory 504 that are updated as each beat ofdata is popped from the FIFO 508. AWC logic (not shown) examines thecurrent address counter, AXI LENGTH (AWLEN), and AXI SIZE (AWSIZE)encoded in the AXI write request on bus 408 to determine if it isappropriate to merge the Write Data and Write Strobe information fromthe current beat with that of previous beats into a common quad-word, orto allocate a new WTAG index.

A parity generator 507 calculates parity over the merged Write Dataquad-word discussed above. The Write Data and corresponding parity aremerged and written to the WB write data memory (WDM) 520 addressed bythe WTAG index and the offset from a per-WTAG context pointer frommemory 504. Similarly, the merged AXI write strobes associated with AXIwrite data in FIFO 508 are written to the WB Write Strobe Memory (WSM)522. The PCC 540 sends a write command acknowledge to the WB 410 when itreceives, via bus 529W, a combined response over channel 428. The writecommand acknowledge on bus 529W from PCC 540 includes a last writecommand done status as well as the WTAG index of the combined response.Also included on the bus 529W is the status as to whether the combinedresponse indicates an acknowledge or an error response. The tapped pipe509 monitors the write strobe portion of the write data from channel 406for unacceptable write strobe patterns as defined in the AXIspecification referred to above. AWC 502 combines illegal write strobepattern status information and unacceptable on channel 408 write commandrequests to create bogus generated write command responses. The bogusresponses are necessary in order to provide a means to respond to AXIWrite requests on channel 408 for which there will be no correspondingcomplete PLB write command. AWC 502 arbitrates by interpolating boguslygenerated write command responses between write command ack responsesfrom PCC 540. AWC 502 combines the combined response error and WTAGindex status information with the bogus responses to provide a formattedresponse to the B FIFO 512 for delivery to the AXI BRESP channel 426.The BID for the BRESP is looked up using the response WTAG indexprovided by the alternate sources PCC 540 or AWC 502 for use as anindex, the PCC 540 as the source when the response is to be an “okay”(an acknowledged combined response), and the AWC 502 as the source whenthe response is to be a “slverr” (an errored combined response or anunsupported (bad) request).

The PWC 530 controls the PLB write data channel 412. For acknowledgedPLB write commands, the PWC 530 loads the WTAG index for retrieval ofthe request corresponding to that command. The PWC 530 uses that WTAGindex to retrieve the request information from per context registers 526and memory 520, requests the PLB write data channel 412 and, whengranted access, supplies that data to the PLB write data channel 412.

The PLB Command Control (PCC) block 540 issues commands over the PLBcommand channel 414 and receives responses on the PLB combined responsechannel 428. When the combined response indicates an read or writecommand acknowledgement from the PLB controller 404 over bus 428 hasbeen received, a write command acknowledge on bus 529W is presented tothe PLB Write Control (PWC) 502 for a write command, or a read commandacknowledge on bus 529R to the Read Combined Response Logic block 582for a read command, as appropriate. In addition, for a write, the WTAGindex is passed to the WTAG_D FIFO 528 via bus 529W after a writecommand acknowledgement is received.

If the AXI requests a read transaction, but there is a PLB error, theAXI read data channel requires a response consisting of the number ofbeats requested, and an error response on channel 426 for each generatedbeat on channel 424. In response to the read command acknowledgement onbus 529R, a bogus beat generator 551 in the PRC block 550 generatesdummy data for transmission to the AXI bus over channel 424.

The RTAG index (during a read) or the WTAG index (during a write) issent to the PLB in a bit field of the PLB command over channel 414. Theindex is returned by the PLB on each PLB beat of the read data transferburst on channel 422 for a read command. The PRC block 550 recovers theRTAG index and passes it into the read Data FIFO 584 along with theparity and the read data. The WTAG index for PLB write commands mightnot be returned by the PLB and, thus, might not be used. The bogus beatgenerator 551 also stuffs RTAG indexes for the bogusly generated beatsinto the FIFO 584. Because the PLB read data phase is not flowcontrolled and does not forgive overruns, an arbiter (not shown)multiplexes the bogus beats with the PLB beats. Thus the read data FIFOdata stream can have interlaced beats that are bogus and not bogus.Also, because of splitting, read data from one AXI request might be fedto the PRC 550 immediately back-to-back following a portion of anotherAXI requests read data before either are complete. To handle thissituation, the ARC 570 reads FIFO 584 and uses temporary RTAG indexes tolook up the RID from the read request memory 574.

If the response on channel 428 is a retry response to a command, the PLBTimer Control (PTC) 560 will provide a pseudo-randomly determined delayperiod after which it will communicate to the PCC 540 that the commandis to be retried.

The PLB hang Detect Control (PHC) block 534 provides a mechanism forensuring forward progress on outstanding commands and otherwise assertsa hang detect signal. The PRC block 550 accepts valid data after bitswapping in block 552 from the PLB channel 422 in response to previouslyacknowledged read commands. The PRC 550 must be able to accept a beat ofsuch previously requested data on every clock. This data may be storedin the RB 418, later to be retrieved by the ARC 570 and delivered to theAXI RDATA channel 424.

The PLB Write control (PWC) block 530 requests the PLB write data bus412 after receiving a write command acknowledgement response throughFIFO 528 or from a faster more direct path from the PCC 540 (not shown).When the PLB write data interface 404 grants the write data bus 412, themaster must send a burst of write data stored in the write data memory520. The PWC 530 posts write completion status over bus 532 to the AWC502, which de-allocates the write request tag therein. Completion ofdelivery of a context's write data to the PLB channel 412 could be manyclock cycles later than the write request terminating event thattriggers the pushing of the BRESP to B FIFO 512. The WTAG indexedcontext thus may persists far longer than the write request.

The AXI Read Control (ARC) block 570 interfaces to the AXI slave readchannels 416, 424, to accept requests, and communicates read channeldata and responses to ARC 570. Clock domain crossing, rate adaptation,and some buffering are provided by the AR and R FIFOs in ARC 570 for theRADDR (416) and RDATA (424) read channels. The Read Request Memory 574stores AXI Read Channel requests by RTAG-indexed entries. In thisembodiment, memories 574 and 504 are indexed with values ranging fromzero to fifteen. Thus both memories are divided into sixteen portions.Each portion of memory 574 represents a potential read request trafficcontext. In the read path, the RTAG index is associated with datareceived over channel 422. Thus, the RTAG index might change frombeat-to-beat and may be intermingled with other read request PLB beatsand bogusly-generated bad beats from PRC 550. Since PLB beats arequad-words, and AXI beats might be less than quad-words, the pipeline578, when appropriate, repeats PLB data beats into the R FIFO 584, anduses the RTAG index to look up the RID from an array of ARIDs, using theRID Lookup sub-block in ARC 570 operating on information stored inmemory 574. Also for each AXI beat, the AXI size and length is needed togenerate a flag indicating the last beat of the burst is being profferedon channel 424. In addition, the parity of the PLB beats is checked inparity check block 579 and any parity errors are reported beat by beatin the response on bus 426 if parity checking is enabled. AXI readrequests on channel 416 are effectively flow controlled by the RTAGreservation and ARID and RTAG vacancy checks within 576. If there is novacancy in the RTAG memory 576, or a repeat ARID is detected, the AXIrequest is not popped from FIFO 572.

As will be explained in more detail in connection with FIG. 6, when anew read request is presented to the ARC 570, the vacancy status of theread request memory 574 is checked for a free RTAG index, and for thenew command's ARID having been already present. If the ARID is alreadypresent, or if there are no free RTAGs, the request must wait in the ARFIFO 572 for the corresponding RTAG entry to be de-allocated.

The AXI Read Control (ARC) block 570 interfaces to the AXI slave readchannels 416, 424 to accept read requests, read address and to deliverread data and read response status to the AXI bus 304. As explained inmore detail below, the AR, and R FIFOs in ARC 570 provide clock domaincrossing, rate adaptation, and some buffering for the RADDR (416), andRDATA (424) read channels. The Read Request Memory 574 stores AXI ReadChannel requests by entries indexed by RTAG indexes from ARID andvacancy check block 576 operating on previously stored contexts inmemory 574. The memory 574 may be implemented as a group of registerbit, vector, and counter arrays and is a memory indexed, in thisembodiment, with values ranging from zero to fifteen. Thus, the memory574 is divided into sixteen portions. Each portion represents apotential read request traffic context comprising an AXI read requestfrom channel 416, the beats of read data and the status and progress ofthe request channel 424. Different requests can simultaneously proceedthrough various pipeline states from clock cycle to clock cycle. As willbe explained in more detail in connection with FIG. 6, when a new readrequest is presented to the ARC 570, the vacancy status of the readrequest memory 574 is checked by block 576 for a free RTAG index, andfor the new command's ARID (comprising a command ID from an AXI request,one portion identifying the AXI master, the other portion used by theAXI master to distinguish one of its requests from another having beenalready present). If the ARID is already present, or if there are nofree RTAG indexes, the request must wait in the AR FIFO 572 for thecorresponding RTAG entry to be de-allocated. Read data is popped from584 to the RDATA pipe 578. The RDATA pipe accommodates the smaller beattransfer sizes by repeating the quad-word sized beats popped from FIFO584. The read request memory includes counters and state variableregisters tracking the requests current address, beat count and per RTAGbit mapped valid status. The RDATA pipe 578 also generates the last beatof burst status as part of the burst response and detects requestcompletion. Upon request completion the valid bit indexed by the RTAG ofthe previously admitted request is cleared making that RTAG value onceagain available. Quad-word beats popped from FIFO 584 can be interlacedand intermingled with other requests. The ARC RID lookup in block 576uses the RTAG requests popped with each beat from FIFO 584 to index therequests ARID to obtain each beat's RID. Parity checking of each beat isperformed by parity check block 579 as data enters the RDATA pipeline578. If the beat's parity enable flag from FIFO 584 was set, and theparity check fails, each beat delivered to FIFO 586 will be marked ashaving a parity error (“slverr”) rather than with an “okay” response.Also, beats bogusly generated by generator 551 will bear this errorresponse marking having been so tagged when pushed into FIFO 584. TheRTAG vacancy check mechanism in block 576 provides flow control as FIFO584 is sized large enough to accommodate sixteen complete responses,this being necessary as channel 422 cannot otherwise be flow-controlledonce PLB read commands have been issued from PCC 540.

The PLB typically operates at a higher clock rate than the AXI bus.Clock rate adaptation is performed in the AXI Write Control (AWC) 502and AXI Read Control (ARC) 570 blocks. Much of the clock rate adaptationis handled by the various FIFOs in these blocks. Thus, most of the logicin the bridge 306 operates at the PLB clock rates.

Exemplary Read Operation of A2P Bridge 306

When a read command from the AXI bus 304 is received, the bridge 306performs the following exemplary operations:

A. Read requests over the AXI Read Address (RADDR) channel 416 isbuffered by a FIFO 572 and is loaded into the FIFO 572 if there is roomto do so. If there is no room or, optionally, when the FIFO 572 is“almost full”, the read request is not acknowledged by the ARC 570 tothe requesting client via to the AXI bus 304.

B. The ARC 570 reads an entry from the AR FIFO 572 and compares its ARIDto outstanding ARIDs in the RTAG memory 576. If there is no match, and avacancy is available, a four bit RTAG index is assigned to the newrequest. Sixteen potential RTAG contexts each representing a completeAXI Read request can be accommodated. The Read Data FIFO Memory 584 islarge enough to accommodate, in this embodiment, a maximum number ofsixteen beats from PLB channel 422 for every read request's trafficburst.

C. The Read Request from the FIFO 572 is posted to the Read RequestMemory 574. The request memory holds the ARID and other per RTAG contextinformation (e.g., state variable and state counter information)associated with the AXI read request. This information is sent with thePLB read command and returned in the PLB combined response, and is usedto associate PLB beats with AXI read requests and to use the RID lookupblock 587 looking up RIDs and beat-by-beat context status from theRTAG-indexed read request memory 574.

D. The ARC 570 posts a read request to the RB RTAG_C FIFO 580.

E. The PLB Command Control (PCC) 540 reads entries an RTAG from theRTAG_C FIFO 580.

F. The PCC 540 presents the read command to the PLB Bus controller 404via channel 414.

G. The PCC 540 receives the combined response from the Bus controller404.

H. The PCC 540 updates the read combined response logic 582 with any oneof “rsv” (an error condition that infrequently occurs), “ack”(acknowledge and accept the command), “retry” (PLB arbiter 404 in FIG.4) indicates the system is not ready to accept the command at this time,command is rejected and needs to be retried), or “err” (error, commandis rejected, no data tenure will occur).

For the combined responses of “rsv” or “err”, a per RTAG status registermust be marked with a bad status corresponding to the offending RTAGindex. Although no PLB data tenure will occur, an error-flagged responseto the AXI Response channel 424 must be generated. This marked badstatus is passed along with other bad request status information to thebogus beat generator 551.

For combined responses of retry, PCC 540 pushes the RTAG index onto itsretry FIFO (not shown). For combined response of acknowledge, the PCC540 must update its RTAG indexed split status to indicate whethersubsequent PLB read commands need to be generated to satisfy this RTAGindexed request entry.

I. The PRC 550 receives the read data from the PLB via channel 422 afterbit reordering in swapper 552.

J. The PRC 550 copies the read data to the read data FIFO 584.

K. The RTAG information is updated by logic (not shown) in read buffer418 on each beat of the read data burst from the PLB channel 422.

L. The ARC 570 transfers data and RTAG indexes from the read data FIFOmemory to pipeline 578 in ARC 570.

M. Parity is checked in the R Data Pipeline 578. The ARC 570 uses theRTAG as an index to look up the Read ID (RID), a bit-field in thechannel 424, and the other information stored at the RTAG index to theRead Request Memory 574 and per RTAG context state variables to properlyformat the bursts for delivery into the ARC R FIFO 586. When the burstis completely delivered to the R FIFO 586, the associated RTAG isde-allocated from RTAG memory 576. The ARC delivers the read data fromthe R FIFO 586 onto the AXI RDATA channel 424 in the same order as itwas received from the PLB 310. In this embodiment, the AXI read databursts on channel 424 could be interleaved since AXI transactions can besplit into multiple PLB transactions.

Exemplary Write Operation of A2P Bridge 306

When a write command from the AXI bus 304 is received, the bridge 306performs the following exemplary operations:

1. Write requests from the AXI Write Address (AXI WADDR) channel 408 arereceived by the AXI Write Control block (AWC) 502 and the requests arestored the AW FIFO 514. The AXI interconnect fabric 402 is controlled bylogic within the AWC 502 that accepts address and control informationinto the AW FIFO 514 over channel 408 whenever there is room in thatFIFO. The AWREADY signal, one of several control signals from the AXIWDATA channel 406, is asserted when there is room in the FIFO, butde-asserted when the FIFO is Almost Full.

2. The AWID and vacancy check block 510 in the AWC 502 reads an entryfrom the AW FIFO 514 and compares its AWID to outstanding AWIDs inmemory 504. As will be explained in more detail in connection with FIG.6, if there is no match and a vacancy is available, a four bit WTAGindex entry in the WTAG-indexed memory 504 is assigned, reserving memoryspace in the Write Request memory 504, Write Data memory 520, WriteStrobe memory 522, and STAG registers 526. Write Data Memory 520 isallocated in blocks large enough to accommodate the maximum AXI writedata burst of sixteen beats.

3. The AWC 502 writes the request to the Write Request Memory 504. Thisincludes a list of the AWIDs and other information from the AXI writerequest. The AWC 502 keeps track of per WTAG context state variables inlocal registers in the AWC 502. Here, a context comprises an AXI requeston bus 408, a PLB command on bus 414, PLB and AXI data transfers overtheir respective buses, associated responses, TAGS, state variableregisters, and how signals therein propagate through the variouspipelines. A context commences with an AXI request and although therequest terminates with an AXI response on channel 426, the contextpersists until completion of write data delivery on channel 412 and theAWC 502 is notified of the completion via bus 532. Thus, the writecontext may persist many clock cycles following the write requestcompletion which terminates with the pushing of the BRESP to FIFO 512.

4. The AWC 502 receives Write data from the AXI Write Data (AXI WDATA)channel 406 and stores the data into the W FIFO 508. The Write Data (AXIWDATA) channel 406 is controlled by logic that accepts data into theFIFO 508 whenever there is room in the FIFO. The WREADY signal, acontrol signal included with the AXI WDATA channel 406, is asserted whenthere is room in the FIFO, but de-asserted when the FIFO is almost full.

5. The AWC 502 reads entries from the W FIFO 508 and compares the WID tothe outstanding valid context AWIDs. If there is a match, the AWC usesthat WID to lookup the beat's WTAG index. It then uses that WTAG indexto access local state variable registers in the WID check block 506 fora merge operation. It posts the merged beat to the WB Write Data Memory520 using the WTAG index and the WTAG indexed per context pointerregister as the Write Data Memory Address.

6. When WLAST, a control signal originating from the AXI channel 406,has been popped from FIFO 508, the AWC 502 posts an entry from pipeline509 to the WB WTAG_C FIFO 524 indicating that the last AXI beat onchannel 406 has been received by FIFO 508.

7. The PCC 540 reads entries from the WB WTAG_C FIFO 524 to retrieve aWTAG index to the AXI Request and Data Memory 504. From the perspectiveof the PCC 540, this is a newly ready and available request for which aPLB command is to be issued, and has not been previously issued, splitor retried. The PCC block 540 receives both read and write requestsRTAG_C FIFO 580 and WTAG_C FIFO 524, respectively. The PCC arbiter 541decides which if any request should be issued, and presents therequest's tag to the command pipeline 542. The PCC pipeline 542 thenissues commands to the PLB controller 404, and receives responses tothose issued commands on the PLB Combined Response channel 428.

The PCC arbiter 541 chooses from several sources. New read and writerequests are popped from either the RTAG_C FIFO 580 or the WTAG_C FIFO524 respectively. Similarly, indices for read and write splits (where anAXI request cannot be accomplished with a single PLB command) are poppedfrom either the r splits or w_splits per tag bit mapped registers (notshown) in PCC 540. Commands which have been previously issued, but thatneed to be retried, are popped from a retry FIFO (not shown) in PCC 540.Alternately, a null tag can be selected if no command should be issued.

The PCC 540 uses the indices selected by the arbiter 541 to retrieve therequest information from either the Read or Write request memory 574,504, respectively. The command including generated address and byteenable parity is written to the PCC command pipeline 542.

When the PLB command channel 414 is ready, the PCC 540 delivers thecommand request over channel 414 and advances the pipeline 542. The PLBallows the bridge 306 to issue up to two commands for which combinedresponses have not been provided. The bridge 306 complies with thisrequirement using an up down credit counter (not shown) that isinitialized to the number of allowed outstanding PLB commands and whichcounts down when the bridge 306 issues a command and up when a combinedresponse is received. The credit counter is located in PCC 540.

The indices for each issued command are pushed into the PCC 540'sresponse FIFO (not shown). When the combined response is received, anentry from the response FIFO is popped. The entry popped from theresponse FIFO thus represents the read or write request to which thecommand's combined response applies. The read bit indicates whether thetag represents a read or a write.

8. The PCC 540 presents a write command to the PLB controller 404 viachannel 414. When the combined response from controller 404 indicates anacknowledgement over channel 428, and the acknowledged command was awrite command, an entry is pushed into the WTAG_D FIFO 528.

9. The PCC 540 receives a combined response from the Bus controller 404via channel 428.

10. When the final combined response for the request's last PLB commandis received over channel 428, the PCC 540 sends the write responsestatus and WTAG index value to the B FIFO 512 in AWC 502 via bus 513.

11. The BID lookup block 505 in AWC 502 uses the write response WTAGindex (hereinafter “BTAG”) to look up the BID, the AXI write responsechannel 426 identification number (ID) associated with the ID of thewrite command received over channel 408. The BTAG, is used to index theAWID from memory 504. The AWC 502 formats an AXI Write response,indicating that a write has been completed by the PLB controller 404,and stores it into the B FIFO 512.

12. AWC interface control logic (not shown) within AWC 502 delivers theAXI Write response from the B FIFO 512 to the AXI BRESP channel 426.

13. The PCC 540 updates the WB registers 526 with the PLB Slave Tag andPLB Slave ID for a valid combined response to the PLB command channel414. It also enters the WTAG index into the WTAG_D FIFO 528. The slavetag is given to the bridge 306, as a PLB master, as part of the combinedresponse on channel 428 to a write command. The slave tag originates inthe slave (within PLB subsystem 310 in FIG. 4) targeted by the PLBmaster and is passed through the PLB Bus Controller 404 to the bridge306. The bridge 306 provides the slave tag along with the data in thePLB write data transfer phase. This allows the target PLB slave to matchthe transferred data with the earlier command. The slave ID is given tothe master as part of the combined response over channel 428 to a writecommand. This slave ID originates in the PLB controller 404. The bridge306 provides this slave ID along with the data in the PLB write dataphase, so that the PLB controller can route the write data to thecorrect target slave.

14. The PWC 530 reads the WTAG index from the WTAG_D FIFO 528, requestsand is granted the PLB write data channel 412 and reads the Write DataMemory 520 and Write Strobe Memory 522 at the WTAG index for abeat-by-beat retrieval of stored and merged AXI Write Data in memory 520and stored merged AXI Write Strobes and parity in memory 522. Thisinformation is, in the next step, transmitted to the PLB controller 404over the PLB write data channel 412 after bit reordering in swapper 536.

15. The PWC 530 sends the burst to the PLB controller 404 over channel412. When the PWC 530 completely delivers the last burst, it signals theAWC 502 via bus 532 to de-allocate the corresponding WTAG index inmemory 504.

The PLB write data channel 412 includes control signals, write data,write data parity, slave ID (SID) and the slave tag (STAG). The SID andSTAG accompany the write command's combined response acknowledge and arereceived from the combined response interface 414. The SID and STAG arestored to, and retrieved from, per context registers 526 in the WB 410.The SID is supplied with the PLB write data access request. The STAG isprovided with each beat of data traffic sent over channel 412.

As described above, the above embodiment handles transactions from asource, such as the AXI bus, which uses a source ordering ID andsatisfies each source transaction with one or more transactions on adestination, such as the PLB 310 (FIG. 4), which supports transactionretries. This embodiment allows a subset of the source ID bits (AWID)and the data being accessed by a transaction to be used to determine theorder that destination transactions are issued. By not doing so,conflicting transactions from one or more AXI bus masters might causeoverwriting of application-sensitive memory-mapped locations accessiblethrough the PLB 310.

It is desirable that write request (command) ordering is enforced usinginformation, such as address and straddle status stored in registerarrays (not shown) in memory 504, other than the AXI ID. The WID and BIDvalues are looked up using the appropriate WTAG index to the AWID arrayin memory 504.

Additionally, the master portion of the AXI ID, unique to the master,may be considered separately from the transaction portion of the AXI ID,particularly if multiple requests are coming from a single AXI busmaster. It may also be advantageous to consider AXI read and writerequest address straddling of cache line boundaries in the PLB whenaccepting or refusing a request.

As described herein and in one exemplary embodiment, the AWC 502 is ableto take into consideration the candidate AXI request's beginning memoryaddress (AXI address field), amount of data (from AXI Length and AXISize fields) to determine if there is straddling of the 128 byteboundary requirement of the PLB, in addition to the AXI ID field, beforea write tag is granted. Further, stored information derived frompreviously admitted AXI requests may also be taken into consideration,as well as the history of the progress (stored in progress counters inPHC 534) in processing those requests, e.g., the latency and number ofretries (stored in retry counters (not shown) in PTC 560) of similarrequests. A block diagram of an apparatus that provides theabove-described functionality is shown in FIG. 6.

FIG. 6 shows a portion of the AWC 502 in FIG. 5. It desirable that theproper ordering of AXI write requests is verified before the writerequest are stored in the write request memory 504. For data coherency,write requests with the same AWIDs should be sent to the PLB controller404 in the same order in which the requests were received and in theorder in which the PLB write command sequences from PCC 540 areacknowledged from PLB controller 404. To do so, the AWC 502 preservesthe ordering of writes by blocking new write requests that are withinthe same write request ID/Address region. In this embodiment, block 510matches the AWIDs of previously admitted requests stored in memory 504to AWIDs from candidate write requests from channel 406 and stored inFIFO 514. Thus, AXI write requests with the same AWIDs are completedsequentially in the order presented to AWC 502.

As described above in connection with FIG. 5, the AWID block 510 obtainsa write request from the AW FIFO 514 and compares the AWID of therequest with AWIDs 622 in the WTAG-indexed memory 504. If no matchoccurs and there is room in the WTAG-indexed memory 504, the AWID of therequest is entered into the WTAG-indexed memory 504 and a corresponding“valid bit” 602 is set corresponding to the entered AWID. The AWID hasboth a master portion 640 and transaction portion 642 used forperformance monitoring as described below in connection with FIGS. 7 and8. The master portion 640 identifies the particular AXI master issuingthe command, and the transaction portion 642 identifies a particulartransaction from that AXI master.

As configured by control bits from the GCS 440, the AWID and vacancycheck block 510 matches AWID and/or write address AWADDR of pendingwrite requests at output 624 of FIFO 514 to AWIDs and AWADDRs forpending write requests in memory 504. Should no match occur and there isroom in memory 504 (when at least one of the Valid bits 620 is not set),then output 630 pops the candidate request from the FIFO 514 and loadsthe candidate write request into the memory 504, illustrated by theclosing of switch 632. Alternatively, the block 510 may be configured tomatch just the master portion 640 of the AWID 622.

Completion of a pending request's context is indicated by thecorresponding Valid bit 620 at the WTAG position in the bit-mappedmemory 540 not being set. Completion of requests is simultaneous withthe pushing of an entry into B FIFO 512. However, the clearing ofWTAG-indexed Valid bits 620 does not occur until delivery of write datato the PLB is complete.

Write requests that straddle 128-byte boundaries have a “Straddle” bit(S) 642 set. If a write request that straddles a 128-byte boundary isadmitted (after all requests with the matching master ID have beensatisfied and their WTAG released), the corresponding S bit 642 is sentand any subsequent request with the same Master portion 640 of the AWID622 will be blocked because the corresponding S bit entry is set for theentry that matches that master ID. Here, the straddle check isimplemented in hardware. If the value of an AXI request's width adjustedAWLEN (part of the AXI command sent over channel 408) is greater thanthe complement of the quad-word addressing portion of that same AXIrequest, AXADDR, then a straddle condition is declared. For example,should an AXI subsystem master in the AXI bus subsystem 304 (FIG. 4)vary the non-master portion of an AXI AWID it generates, write orderingis still desirable. If the master portion of the AXI AWID for the AXImaster is specified, and there was an additional requirement that theconfigured master portion of the AXI AWID also be matched, other“non-offending” AXI subsystem AXI masters might be spared theperformance penalty of unnecessarily blocked requests.

The foregoing discussion regarding candidate write requests processed byblock 510 is also applicable to read requests received over channel 416from the AXI subsystem 304 for processing by ARC 570. While notdescribed in further detail herein, it is understood that the ARID andvacancy check block 576 functions substantially the same as the AWID andvacancy block 510, for candidate read requests in AR FIFO 572 prior towriting into buffer read request memory 574.

In an alternative embodiment, a set of DCR-accessible programmableapplication master ID registers can be optionally programmed with listsof master ID values, or ranges of master ID values that must also bematched when the transaction ID portion 642 of the AWID 622 is notconsidered. In addition, a set of DCR-accessible programmableapplication master ID registers (not shown) can be optionally programmedwith lists of AXI address values, or ranges of AXI address values thatmust also be matched to satisfy a match.

In still another embodiment, the addresses provided by a command sentover the AXI WADDR channel 408 or AXI RADDR channel 416 are checked tosee if the addresses span reserved or protected addresses on the PLBsubsystem 310. For example, critical boot code, encryption keys, messageauthentication keys stored in memory coupled to the PLB, or otherhardware, such as input-output ports, control registers, etc.,accessible from the AXI bus through the bridge 306, should be protectedfrom unauthorized or inadvertent access or overwriting through the AXIbus.

If the addresses being accessed are reserved or protected, then it isusually desirable that the read or write command from the AXI bus bedenied, i.e., not conveyed to the PLB subsystem 310 and, instead, anerror message returned to the AXI bus client originating the command.However, it is understood that there are certain AXI bus clients thathave permission to access these special memory locations. Thus, checkingof the source of the AXI command is needed, in addition to addresschecks, to decide to allow or deny a particular AXI command.

An exemplary apparatus that vets AXI write requests is shown in FIG. 9.A similar apparatus for vetting AXI read requests is shown in FIG. 10.

FIG. 9 shows a portion of the AWC 502 and GCS 440 in FIG. 5 that worksin cooperation with the AWID and vacancy check block 510 and writerequest memory 504 shown in FIG. 6.

As will be described in more detail below, a write access table 902provides address, identification, and other fields, along with controlflags, that are used by write access security check (WASC) block 904 tocheck whether or not a candidate AXI write requests (commands) has the“permission” to access certain addresses in the PLB subsystem 310 (FIG.4).

The write access table 902, preferably a register file (not numbered)within GCS 440 (FIG. 5), has entries that are preferably accessed byprivileged configuration transactions via channel 434 from a hostprocessor or the like, preferably within the network processor 100 (FIG.3). Each entry in table 902 may include various fields to specify aspecial access condition. Preferably, there are multiple entries in thetable 902. In this exemplary embodiment, each entry include a targetaddress field (TADDR), command special master identification field(SMID), first and second flags (ALLOW, DENY), and an optional specialmaster user ID (SMUID) bit field, as well as masks corresponding to thevarious fields. Each mask allow for a conventional per-bit masking ofbit-by-bit matching of bits in the field from the FIFO 514 and thecorresponding field in the entry, so that addresses and commandidentifications can span a range of possible values for a match.

Similar to that described as in connection with FIG. 6, output 624 ofthe FIFO 514, for a write request received from AXI bus subsystem 304(FIG. 4) via channel 408, comprises the write request ID (AWID) field622, a write address (AWADDR) field, and an optional user ID (AUID)field. As described in more detail in connection with FIG. 11, WASC 904compares the TADDR entry in table 902 to the AWADDR on output 624, andthe mask, TADDRM, applied to the result to allow a range of addresses tobe matched. In an another embodiment, AWADDR, AWID, and AWLEN (discussedabove) from output 624 would undergo the address straddling check,similar to that performed by block 510 as described above, such addressstraddling for specified masters would be conditionally disallowed toprevent skirting of access privileges enforced by block 904. In anotheralternate embodiment, multiple entries in table 902 have differentaddresses TADDR to provide address range guard bands to prevent skirtingof access privileges enforced by block 904 through address straddling.

Assuming an address match occurs, the ALLOW and DENY flags are used incombination with ID matching between the AWID from output 624 and theSMID in table 902 (after masking the result using mask SMIDM) todetermine if the candidate write request will be allowed or not. As willbe described in more detail below, if the ALLOW flag for the entry isset, only those AXI masters in the AXI bus subsystem 304 whose SMID andSMIDM fields when applied to the AWID from output 624 would be allowedaccess to the address range specified by that table entry's addressrange specification. If, however, the DENY flag for the entry is set,only those masters whose SMID and SMIDM fields when applied to the AWIDfrom output 624 would be denied access to the address range specified bythat table entry's address range specification. If neither the ALLOW orthe DENY flag is set, then no action by block 904 will occur. Forpurposes here, having both the ALLOW and DENY flags set is an “illegal”configuration but alternative methods of utilizing this “illegal” stateis discussed below.

Failure of write requests to satisfy such a match restriction would,upon admittance of the candidate write request into memory 504 by block510 as described above in connection with FIG. 6, result in the settingof an additional flag in memory 504, a Write Access Security CheckFailure (WASCF) flag 910, by output 908 for that particular candidatewrite request. The setting of the WASCF flag preferably causes thereturn of a error acknowledgement to the AXI bus subsystem 304 viachannel 426, and no PLB command is generated by PCC 540 (FIG. 5).Optionally, the failure will result in the generation of an interrupt905 to advise an external host processor or one of the processors 114₁-114 _(M) (FIG. 3) of the violation. Should the candidate write requestsatisfy the address and ID requirements, then the WASCF flag 910 is notset.

As mentioned above, there might be multiple entries in the access table902. Assuming so, then the block 904 sequences through the entries tofind address and ID fields that match those of the candidate request.While it may be desirable to sequence through all of the entries, it isunderstood that sequencing might be stopped once the address and ID bothresult in a match.

Similar to that discussed above in connection with the vetting ofcandidate write requests described above, output 1012 of the AR FIFO572, for a read request received from AXI bus subsystem 304 (FIG. 4) viachannel 416, comprises the read request ID (ARID) field, a read address(ARADDR) field, and an optional user ID (AUID) field. As described inmore detail in connection with FIG. 11, RASC 1004 compares the TADDRentry in table 1002 to the ARADDR on output 1012, and the mask, TADDRM,applied to the result to allow a range of addresses to be matched. In ananother embodiment, ARADDR, ARID, and ARLEN (equivalent to the AWADDR,AWID, and AWLEN as discussed above but for read requests) from output1012 undergo the address straddling check, similar to that performed byblock 510 as described above for write requests, such address straddlingfor specified masters would be conditionally disallowed to preventskirting of access privileges enforced by block 1004. In anotheralternate embodiment, multiple entries in table 1002 have differentaddresses TADDR to provide address range guard bands to prevent skirtingof access privileges enforced by block 1004 through address straddling.

Assuming an address match occurs, the ALLOW and DENY flags are used incombination with ID matching between the ARID from output 1012 and theSMID in table 1002 (after masking the result using mask SMIDM) todetermine if the candidate read request will be allowed or not. As willbe described in more detail below, if the ALLOW flag for the entry isset, only those AXI masters in the AXI bus subsystem 304 whose SMID andSMIDM fields when applied to the ARID from output 1012 would be allowedaccess to the address range specified by that table entry's addressrange specification. If, however, the DENY flag for the entry is set,only those masters whose SMID and SMIDM fields when applied to the ARIDfrom output 1012 would be denied access to the address range specifiedby that table entry's address range specification. If neither the ALLOWor the DENY flag is set, then no action by block 1004 will occur. Forpurposes here, having both the ALLOW and DENY flags set is an “illegal”configuration but alternative methods of utilizing this “illegal” stateis discussed below.

Failure of read requests to satisfy such a match restriction would, uponadmittance of the candidate read request into memory 574 by block 576 asdescribed above in connection with FIG. 6, result in the setting of anadditional flag in memory 574, a Read Access Security Check Failure(RASCF) flag 1010, by output 1008 for that particular candidate readrequest. The setting of the RASCF flag preferably causes the return of aerror acknowledgement to the AXI bus subsystem 304 via channel 426, andno PLB command is generated by PCC 540 (FIG. 5). Optionally, the failurewill result in the generation of an interrupt 1005 to advise an externalhost processor or one of the on-chip processors 114 ₁-114 _(M) (FIG. 3)of the violation. Should the candidate read request satisfy the addressand ID requirements, then the RASCF flag 1010 is not set.

As mentioned above, there might be multiple entries in the access table1002. Assuming so, then the block 1004 sequences through the entries tofind address and ID fields that match those of the candidate request.While it may be desirable to sequence through all of the entries, it isunderstood that sequencing might be stopped once the address and ID bothresult in a match. Moreover, instead of having separate access tables902, 1002, it might be desirable to have a common access table with anadditional field indicating whether a particular entry in the table isfor write requests, read requests, or is applicable to both requests.

FIG. 11 illustrates an exemplary operation of the blocks 904 and 1004 ina flowcharted process 1100. It us understood that while the flowchartimplies that the illustrative steps are done sequentially, many of thesteps may be done substantially simultaneously as is well known topersons with ordinary skill in the art. Further, the process 1100 isgeneric to both read and write commands, but may be modified forspecific needs, as required. The following description of the flowchartis based on an exemplary write command request received over AXI WADDRchannel 408 but may be applied to read commands over AXI RADDR channel416.

Beginning with step 1102, a candidate request (e.g., a write commandover channel 408, and a read command over channel 416) is received andheld in FIFO 514 (FIGS. 5 and 6) (FIFO 572 for reads) and the error flagcleared (WASCF for write command requests, RASCF for read commandrequests). As explained above in connection with FIG. 6, the candidaterequest has an address field, AWADDR, and an identification field, AWID,associated with the command. In step 1104, the request address field,AWADDR for write requests and ARADDR for read requests, is compared tothe target address TADDR, in the access table 902 for write requests orin access table 1002 for read requests. In step 1106, the requestidentification field, AWID for write requests and ARID for readrequests, is compared to the special master identification, SMID, in theaccess table 902 for write requests or in access table 1002 for readrequests.

In step 1108, if no address match occurred in the comparison step 1104,then control passes to step 1110, where the process 1100 terminates.Because the access tables 902, 1002 may have multiple entries, afterstep 1110 another target address TADDR and corresponding special masteridentification SMID may be checked against the request address andrequest ID in steps 1104, 1106, respectively, and steps 1108-1116, 1122,and 1124 repeated until steps 1118 and 1120 are performed or all entriesin the access table have been checked. Further, a “super allow” flag(not shown) in GCS 440 might be provided to override the settings of theALLOW and DENY flags to permit AXI read and write command requestsunfettered access to the PLB.

Returning to step 1108 and assuming an address match occurred, controlthen passes to step 1112 and the ALLOW flag or bit from the access tableis checked to see if it is set or not. Assuming the ALLOW flag is set,then the DENY flag is checked to see if it is set in step 1114. Assumingthe DENY flag is not set, then the ID match from step 1106 is checked instep 1114 to see if it is set. If it is set, then the command passes itschecks and the command is allowed to proceed as described in connectionwith step 1110. However, if ID match was not successful, then, in step1118, the WASCF flag is set as the command is entered into buffer memory504 (FIG. 9), and an interrupt might be generated in step 1120 to advisean external host processor that a violation occurred. As describedabove, because the WASCF flag is set, the PCC 540 (FIG. 5) does not passthe command to the PLB and forces an error response to be sent over theAXI BRESP channel 426.

Generally, having both the ALLOW flag and the DENY flag to be set is an“illegal” or improper configuration. In step 1114, such a condition willresult in the WASCF flag being set in step 1118 for all commands with anaddress match. Alternatively or in addition, the above-mentioned “superallow” flag (not shown) in the GCS 440 (FIG. 5) may permit an optionaloverride of this condition and such commands are allowed to proceed bycontrol passing to step 1110 instead of step 1118.

Returning to step 1112, if the ALLOW flag is not set and, in step 1122,the DENY flag is not set, then control passes to previously described1110 and the command is allowed to be executed. However, if the DENYflag is set, then the ID match from step 1106 is checked in step 1124.If the ID does match, then control passes to step 1118 and the commandwill not be passed to the PLB, whereas if the ID does match, thencontrol passes to 1110 and the command will be passed to the PLB.

Embodiments of the present invention also provide a method of samplingand accumulating transaction performance data in the bus bridge 306.Performance monitoring capability is made available for both read andwrite transactions in the bridge 306 by using the existing hardware usedin the AWC 502 for write command monitoring, and in the ARC 570 for readrequest monitoring. As shown in FIGS. 5 and 6, the AWID block 510 isused to recognize a portion of the ID of an AXI request. When acomparison between the portion of the AXI ID and a non-negative fixedvalue (i.e. zero) is satisfied, write event monitor start signal 720 isasserted, the entire AWID is stored in memory 504 and the AWC 502 mayenable a performance monitoring that it ignores subsequent requests forstart event triggering with the same AWID. Another comparison circuit inBID lookup block 505 that monitors the AXI response and triggers when itrecognizes the AXI response ID matches that of the previously storedstart event recognized request, and resets the AWC 502 to thenon-blocking start event monitor state. For a write request, twosignals, write event monitor start (720 in FIGS. 6 and 7) and writeevent monitor stop signal 722 (not shown in FIG. 5 but originates inblock 505) each become asserted for one clock cycle corresponding to theevent recognitions. As discussed below, these signals are monitored by aset of DCR channel-accessible statistics registers capable of latencymeasurements and more. Advantageously, the AWID block 510 initiatessampling of transactions by matching only a portion of the AXI ID of atransactions request, while using the entire ID to recognize thetransaction's matching AXI response to terminate the sampling of a giventransaction.

Similarly, for read commands, a read event monitor start signal 716 (notshown in FIG. 5 but originates in block 576) is generated when thematching of a portion of the candidate request's ARID matches anon-negative fixed value (i.e. zero), while a read event monitor stopsignal 718 (not shown but comes from block 578) is generated when thelast entry in response to a read request is pushed to R FIFO 586 with anRID that matches the entire stored ARID of the request that served tocommencement the monitoring. Alternation between the hunting-capture andcounting/capture blocking state is essentially the same as describedabove for write requests.

It is understood that the non-negative fixed values used for matchingagainst ARID or AWID fields may be programmable via, for example, theGCS 440 (FIG. 5).

As shown in FIG. 7, the statistics (STAT) block 700 provides a means forcounting events and measuring the time intervals over which they occur.The STAT block 700 has seven DCR accessible registers. These includefour general-purpose counters 702-708 that can count any of thepre-wired selectable external events, such as AXI read or writerequests. The clk_counter 710 is adapted to count PLB clock cycles foruse as a chronometric reference, for example. The five counters areprovided with both read and write access via the DCR channel 434. Thus acommand from the DCR subsystem 434 (FIG. 4) may preset or clear them.The control register 712 settings select the events to be counted and,as described below, controls what mode the counters 702-708 operate in.The control register 712 also controls the dynamic starting and stoppingof the counters.

There are two main operational modes of the counters: counting mode andlatency mode. In the counting mode, counting of the selected externalevents monitored over channel 730 may commence when an enable bit in thecontrol register 712 is set. Counting can be halted by setting a haltcounters bit in control register 712. Releasing the halt counters bitallows counting to continue. Counting is also halted when any of thecounters 702-710 overflow, as discussed below.

In the latency mode, the interval between an AXI request with aselectable multi-bit ID field and a response with the same ID ismeasured by counting the number of clock cycles of clock PLB_clk 732,the PLB domain clock, between the request start event and the responsestop event. In this embodiment, counter2 706 keeps track of the currentlatency, counter0 702 holds the maximum clock count historicallyobserved by counter2 706 between start and stop events, and counter1 704holds the minimum clock count historically observed by counter2 706between start and stop events. When the latency mode is chosen, thecontrol register's Write Latency bit (not shown) setting selects betweenmeasuring write latency or read latency. Writes to the control register712 with the latency bit set resets the clock counter 710, counter0 702,counter 1 704, counter2 706, and counter3 708. In this embodiment,counter0-counter3 can be used for other selectable events or purposes,such as counting events over bus 730 and for diagnostic and performancemeasurement.

The counter OVR register 714 contains the overflow bits for the fivecounters 702-710 indicating if one or more of the five countersoverflows. As mentioned above, if one or more of the overflow bitsbecome set, the counting by all the registers 702-710 halts. Register714 can be read, but not written, over the DCR channel 434. The overflowbits in register 714 can be cleared by writing a “zero” to the enablebit in control register 712, followed by writing a “one” to the enablebit. The Scale cnt counter 716 is an internal counter that counts on PLBclock cycles. It can be used to scale the clk_counter 710. A non-zero“scale” bit field in the control register 712 selects the scaling whichdivides the PLB clock by an exemplary eight, thirteen, or sixteen.Otherwise the value of the clk_counter 710 is unscaled. In thisembodiment, the counter 716 is not readable via the DCR channel 434.

For the read latency mode, a read event monitor start signal 716 (notshown in FIG. 5 but originates in block 576) is generated when thematching of a portion of the candidate request's ARID matches a fixedvalue (i.e. zero), while a read event monitor stop signal 718 (not shownbut comes from block 587) is generated when the last entry in responseto a read request is pushed to R FIFO 586 with an RID that matches theentire stored ARID of the request that served to commencement themonitoring. Similarly, for write latency mode, a write event monitorstart signal 720, originating in block 510 as shown in FIG. 6, isgenerated when the matching of a portion of the candidate request's AWIDmatches a fixed value (i.e. zero) when the request is entered intomemory 504, as described above in connection with FIG. 6, while a writeevent monitor stop signal 722 (not shown but comes from block 505) isgenerated when a write request acknowledgment, having a BID that matchesthe entire stored AWID of the request that served to commence themonitoring, is pushed into B FIFO 512.

The measured latency intervals apply to the PLB domain and do notinclude the latency of the AXI interface clock crossing the variousFIFOs in the AWC 502 and ARC 570 (FIG. 5). The blocking featuredescribed above in connection with FIG. 6 advantageously prevents there-occurrence of an additional monitoring process start event (e.g., aread or write request with the same ID) until a stop event has occurred.

After at least one start/stop event cycle has been completed (counter3708 will have a non-zero value), these hardware registers will containthe data necessary for an external processor host or a processorinternal to the network processor 100 (e.g., one or more of theprocessor cores 114 ₁-114 _(M) to compute at least the followingstatistics about the latency seen by the event:

1) Minimum Latency (counter0 702)2) Maximum Latency (counter1 704)3) Number of transactions (counter2 706)4) Total latency (counter3 708)5) Average Latency (counter3/counter2)

Operation of the statistics block 700 is illustrated in simplifiedflowchart 800. To initialize the counters in statistics block 700, instep 802 all of the counters except counter0 702 are set to zero,counter0 being set to all ones. In step 806, when the statistics block700 receives an event monitoring process start signal (e.g., amonitoring an AXI request, such as a read or write, based the request'smulti-bit ID as described above) starts in step 804, the clk_Counter 710is incremented in response to the PLB clock. Until the event stops instep 808, step 806 is repeated. When the event stops, such as upontransmission of a AXI BRESP on channel 426 having the same ID as the AXIrequest that initiated the event in step 804, the monitoring processstops and data regarding the monitored event is determined beginningwith step 810. In step 810, the value of counter0 702 is checked to seeif it is less than the value in clk_counter 710. If so, then in step812, then the value of clk_counter 710 is loaded in to counter0 702 andcontrol passes to step 814. Otherwise, control passes to step 814 wherethe value of counter1 704 is checked against the value in clk_counter710. If the value in counter 1 is greater than that in clk_counter 710,then in step 816 the value of clk_counter is loaded into counter1 andcontrol passes to step 818. Otherwise, control passes to step 818 wherecounter2 706 is incremented and in step 820, the value of counter3 708is increased by the value of clk_counter 710 and control waits untilanother event occurs in step 804. It is understood that one or more ofthe steps described in connection with FIG. 8 may be skipped or notimplemented, and other steps added, as desired.

While AXI and PLB buses are used in the above-described embodiment, itis understood that the bridge 304 may interface other bus types withsimilar characteristics to the AXI and PLB buses. For example, one ofthe primary characteristics of the AXI bus is that reads and writes areseparate and ordering requirements are normally defined by the ID bits.The primary characteristics of PLB, relative to AXI, are the retrymechanism and the difference in data alignment and size of memory accessrequests.

While the exemplary embodiments of the present invention have beendescribed with respect to processing blocks in a software program,including possible implementation as a digital signal processor,micro-controller, or general-purpose computer, the present invention isnot so limited. As would be apparent to one skilled in the art, variousfunctions of software might also be implemented as processes ofcircuits. Such circuits might be employed in, for example, a singleintegrated circuit, a multi-chip module, a single card, or a multi-cardcircuit pack.

The present invention can be embodied in the form of methods andapparatuses for practicing those methods. The present invention can alsobe embodied in the form of program code embodied in tangible media, suchas magnetic recording media, optical recording media, solid statememory, floppy diskettes, CD-ROMs, hard drives, or any othernon-transitory machine-readable storage medium, wherein, when theprogram code is loaded into and executed by a machine, such as acomputer, the machine becomes an apparatus for practicing the invention.The present invention can also be embodied in the form of program code,for example, whether stored in a non-transitory machine-readable storagemedium, loaded into and/or executed by a machine, or transmitted oversome transmission medium or carrier, such as over electrical wiring orcabling, through fiber optics, or via electromagnetic radiation,wherein, when the program code is loaded into and executed by a machine,such as a computer, the machine becomes an apparatus for practicing theinvention. When implemented on a general-purpose processor, the programcode segments combine with the processor to provide a unique device thatoperates analogously to specific logic circuits. The present inventioncan also be embodied in the form of a bitstream or other sequence ofsignal values electrically or optically transmitted through a medium,stored magnetic-field variations in a magnetic recording medium, etc.,generated using a method and/or an apparatus of the present invention.

It should be understood that the steps of the exemplary methods setforth herein are not necessarily required to be performed in the orderdescribed, and the order of the steps of such methods should beunderstood to be merely exemplary. Likewise, additional steps might beincluded in such methods, and certain steps might be omitted orcombined, in methods consistent with various embodiments of the presentinvention.

As used herein in reference to an element and a standard, the term“compatible” means that the element communicates with other elements ina manner wholly or partially specified by the standard, and would berecognized by other elements as sufficiently capable of communicatingwith the other elements in the manner specified by the standard. Thecompatible element does not need to operate internally in a mannerspecified by the standard.

Also for purposes of this description, the terms “couple,” “coupling,”“coupled,” “connect,” “connecting,” or “connected” refer to any mannerknown in the art or later developed in which energy is allowed to betransferred between two or more elements, and the interposition of oneor more additional elements is contemplated, although not required.Conversely, the terms “directly coupled,” “directly connected,” etc.,imply the absence of such additional elements. Signals and correspondingnodes or ports might be referred to by the same name and areinterchangeable for purposes here.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention might bemade by those skilled in the art without departing from the scope of theinvention as expressed in the following claims.

1. In a bridge coupled to a first bus and a second bus, the bridgehaving an access table having at least one entry comprising an addressand an identification value, a method for communicating between thefirst and second buses comprising: A) receiving, from the first bus, acommand having an identification field and an address field; B)comparing the address field of the command against the address in theaccess table entry to generate an address match if the address field andthe address match; C) comparing the identification field against theidentification value in the access table entry to generate anidentification match if the identification field and the identificationvalue match; D) transmitting the received command over the second bus ifthere is an address match and there is an identification match; and E)transmitting an error response over the first bus if there is an addressmatch and no identification match.
 2. The method of claim 1, furthercomprising the step of: F) transmitting the received command over thesecond bus if there is no address match.
 3. The method of claim 2,wherein the access table comprises a plurality of entries, and whereinsteps B) through E) are performed for each entry in the access table andstep F) is performed if no address match occurs for all entries in theaccess table.
 4. The method of claim 2, wherein there are multiplecommand types, there is an access table for each command type, and stepsB) through F) are performed using the access table associated with thetype of command received.
 5. The method of claim 4, wherein the commandtypes comprise are read commands and write commands.
 6. The method ofclaim 4, wherein the command is a read command, the response is a readcommand response, and the method further comprises the steps of: G)receiving data from the second bus in response to the read commandtransmitted in step D); and H) transmitting the received data over thefirst bus.
 7. The method of claim 6, wherein the identification field ofthe read command is transmitted along with the received data in step H).8. The method of claim 6, wherein bogus data is substituted for thereceived data in step G) if an error response is transmitted in step E).9. The method of claim 1, wherein the first bus is an AXI-compatible busand the second bus is a PLB-compatible bus.
 10. The method of claim 1,wherein step D) further comprises the steps of: D1) receiving a responseto the command from the second bus; D2) transmitting the receivedresponse over the first bus.
 11. The method of claim 10, wherein, priortransmitting the command in step D), the received command is placed in abuffer if there is an address match and there is an identificationmatch, the buffered command is transmitted if there is an address matchand there is an identification match, and step D) further comprises thestep of: D3) removing the command from the buffer after performing stepD2).
 12. The method of claim 1, wherein the each entry in the accesstable further comprises first and second flags, and wherein in step D),the command is not transmitted unless the first flag is set and thesecond flag is not set.
 13. The method of claim 12, wherein in step E),the error response is not transmitted if the first flag is not set andthe second flag is set.
 14. The method of claim 13, wherein having thefirst and second flags set is an illegal state and an error response istransmitted over the first bus.
 15. The method of claim 13, wherein ifthe first and second flags are both set, then the received command istransmitted over the second bus if a third flag is set.
 16. The methodof claim 13, wherein if the first and second flags not set, then thereceived command is transmitted over the second bus.
 17. A bridge,coupled to a first bus and a second bus, comprising: an access tablehaving at least one entry comprising an address and an identificationvalue; a receiver, coupled to the first bus, adapted to receive acommand from the first bus having an identification field and an addressfield; a comparator, coupled to the receiver and the access table,adapted to compare the address field of the received command against theaddress in the access table entry to generate an address match if theaddress field and the address match, and adapted to compare theidentification field against the identification value in the accesstable entry to generate an identification match if the identificationfield and the identification value match; a first transmitter, coupledto the second bus and the receiver, adapted to transmit the receivedcommand over the second bus if there is an address match and there is anidentification match; and a second transmitter, coupled to the firstbus, adapted to transmit an error response over the first bus if thereis an address match and no identification match.
 18. In a bridge coupledto a first bus and a second bus, the bridge having an access table witha plurality of entries, each entry comprising an address and acorresponding identification value, a method for communicating betweenthe first and second buses comprising: A) receiving, from the first bus,a command having an identification field and an address field; B)comparing the address field of the command against the addresses in theaccess table entries to generate an address match if the address fieldand at least one of the addresses match; C) comparing the identificationfield against the identification values in the access table entries togenerate an identification match corresponding to the address match ifthe identification field and the identification value, corresponding tothe matched address, match; D) storing the received command into abuffer if there is an address match and a corresponding identificationmatch or if there is no address match for all the entries in the accesstable; E) transmitting the buffered command over the second bus; and F)transmitting an error response over the first bus if the receivedcommand is not stored in the buffer in step D).
 19. The method of claim18, wherein step E) further comprises the steps of: E1) receiving fromthe second bus a response to the buffered command; E2) transmitting thereceived response over the first bus; and E3) removing the command fromthe buffer after performing step E2).
 20. The method of claim 19,wherein there are multiple command types, there is an access table foreach command type, and steps B) through F) are performed using theaccess table associated with the type of command received.